1. Field of the Invention
This invention relates to a weighted golf clubhead which tends to reduce twisting when a golf ball is struck.
2. Description of Prior Art
Most prior golf clubheads may be classified as having either a low or moderate polar moment of inertia as determined along a vertical reference axis: for example, a vertical twist axis axis through the center of mass of a clubhead. For purposes of presentation, the present discussion is confined to putter heads. However, the principles and practices described may be applied to any type of head including woods, irons, and utility clubs.
Any list of classic examples in the low inertia category would include the Cash-In of Spalding and the Bull's Eye of Titleist. These heads may be described as essentially bar-shaped.
Any list of classic examples in the moderate inertia category would include the Pal by Ping and the Inertial by Slotline. These heads may be described as modified bar-shapes. For the case of the Pal, mass is removed from the central region and positioned toward the extremes of toe and heel. In the case of the Inertial, mass is removed from the central region and positioned as higher density material toward the extremes of the toe and heel.
Let us draw a quantitative distinction between the terms low and moderate inertia by considering five-inch, 300-gram systems. A simple bar of this dimension and mass has a moment of inertia of about 4000 g-cm.sup.2. Since in fact both the Cash-In and Bull's Eye have lengths less than five-inches, their actual moments are something under 4000 g-cm.sup.2. Accordingly, any putter head with an inertia up to about 4000 g-cm.sup.2 can be classified in the low inertia category.
On the other hand, both the Pal and the Inertial are available in sizes approximating a five-inch, 300 gram system. Under these circumstances the Inertial has a reported moment of 5,000 g-cm.sup.2. A visual inspection of the Pal compared to the Inertial suggests that the Pal has a moment somewhere in the range of 4000-5000 g-cm.sup.2. Accordingly, any putter head with inertia from about 4,000-6,000 can be classified in the moderate inertia category.
In turn, heads in the range above 6,000 g-cm.sup.2 may be classified as having high inertia. However, inertia is only one of the basic physical characteristics of concern in the design of clubheads. It is also desirable that the center of mass of a clubhead be positioned correctly. Consideration of stability and loft indicate that the center of mass for the clubhead should be placed about midway between the heel and toe, preferably closer to the sole than the top of the clubhead. For most irons and putters the center of mass will be toward the front near the striking face, while for most woods it will be somewhat more rearward toward the midpoint between front and back.
Some examples of at least a potential for difficulty in the simultaneous combination of inertia and center mass may be found in the recent literature. For example, Reiss in U.S. Pat. No. 4,444,395 dated Apr. 24, 1984 described a moderate-to-high inertia clubhead wherein the toe weight was shaped so that its center of mass was in its top portion. As another example, Peltz in U.S. Pat. No. 4,754,976 dated Jul. 5, 1988 described a very high inertia clubhead where the center of mass of the inertial weight was about four inches behind the striking face.